Optical fiber cables, referred to hereinafter as optical cables, usually have one of three types of structure.
In a first type of structure, the optical cable includes a central filamentary strength member around which tubes accommodating optical fibers are assembled in a helical or an SZ configuration. The tube assembly is covered with a sheath. In this first type of structure, the tubes containing the optical fibers are defined by relatively thick and rigid walls of synthetic material. The optical fibers can move relative to the tubes that contain them. Cables having a structure of the first type are described in the documents U.S. Pat. No. 4,366,667 and EP-A-0 846 970, for example.
In a second type of structure, the optical cable includes a single synthetic material tube, usually referred to as an xe2x80x9cuni-tubexe2x80x9d, accommodating optical fibers and, where applicable, tapes, which may be assembled together in a helical configuration. The unitube is covered with a sheath defined by a wall in which filamentary strength members may be embedded.
In a third type of structure, the optical cable includes synthetic material tubes accommodating the optical fibers and assembled together in a helical or an SZ configuration. The assembly of tubes is covered with a sheath defined by a wall in which filamentary strength members are embedded. In this third type of structure, the tubes containing the optical fibers are relatively thin and flexible and they grip the optical fibers that they contain and prevent virtually all relative movement between the optical fibers and the tubes containing them.
In all three types of structure, the synthetic material sheath is usually extruded around what is usually called the optical core; in the first type of structure the optical core comprises the filamentary strength member and the tubes, in the second type of structure it comprises the uni-tube and the strips, if any, and in the third type of structure it comprises the assembly of tubes.
The filamentary strength member is made of steel, for example, or a composite material containing resin and glass fibers. The coefficient of expansion of the strength member is relatively low. On the other hand, the strength member is in longitudinal contact with at least one contiguous member whose coefficient of expansion is relatively high, generally 100 times greater than the coefficient of expansion of the strength member. Depending on which type of structure is used in the optical cable, the contiguous member is a tube containing the optical fibers, and which is generally of polypropylene (PP) or polybutyleneterephtalate (PBTP), or even of polyethylene, in the first type of optical cable structure, or the sheath, which is generally of polyethylene (PE) or polyamide (PA) in the second and third types of optical cable structure.
Consequently, when a cable is subjected to large temperature differences, the difference between the coefficients of expansion of the strength member and the contiguous members causes disparities in the behavior of the various components of the cable.
To remedy these disparities caused by differential expansion in optical cables having a structure of the first or third type, it is possible to reduce the assembly pitch of the helical or SZ configuration of tubes in order to increase the length of the tubes per unit length of the strength member, for example. However, this solution is relatively costly, because shortening the assembly pitch of the helical or SZ configuration of tubes increases the length of the optical fibers per unit length of the cable.
Optical cables having a structure of the first or third type generally include cords for retaining the tubes and these cords can attenuate the differential expansion of the strength member(s) and the tubes. However, the retaining cords have insufficient effect in terms of attenuating differential expansion.
One particular object of the invention is to attenuate effectively differential expansion of the filamentary strength member and the contiguous member(s) in longitudinal contact therewith in an optical cable.
To this end, the invention provides an optical fiber cable including a filamentary strength member made of a material with a low coefficient of expansion and in longitudinal contact with a contiguous member having a high coefficient of expansion, wherein, to increase friction or adhesion between the strength member and the contiguous member, the strength member and/or the contiguous member has striations on its surface in contact with the other member.
According to features of various embodiments of the optical fiber cable:
the strength member is buried in a sheath forming said contiguous member and housing the optical fibers, the strength member having striations for increasing its adhesion to the material of the sheath that surrounds it;
the cable includes an assembly of two or more tubes housing the optical fibers and housed in the sheath;
the tubes are assembled in a helical or SZ configuration inside the sheath;
the cable includes two or more tubes housing the optical fibers and assembled around the strength member, each tube constituting a contiguous member;
the tubes are assembled around the strength member in a helical or SZ configuration;
the strength member has striations to increase friction with the tubes; and
each tube has striations to increase friction with the strength member.
The invention also provides a method of fabricating an optical fiber cable as defined hereinabove, wherein striations for increasing friction or adhesion are formed on the strength member or on the contiguous member using knurling means for knurling said member.
According to other features of the above method of fabricating an optical fiber cable:
the strength member is made of a composite material containing resin and glass fibers and the knurling is applied to the outside surface of the strength member when it has been softened by heating it or the knurling means;
each tube is made of a synthetic material and the knurling is applied to the outside surface of each tube at the exit from a head for extruding the tube;
each tube is made of a synthetic material and the knurling is applied to the outside surface of each tube when assembling that tube with the strength member, with the outside surface of the tube softened by heating it or the knurling means; and
the knurling means include a knurling wheel provided with a peripheral groove defined by a knurled surface adapted to come into contact with either the strength member or the contiguous member.